It is known to employ a gear shroud over rotating bevel gears systems in engine bearing chambers, an example of which is shown in FIGS. 1 to 3. Such gear shrouds are intended minimise the pumping losses from the gears and hence reduce the level of heat generated within the bearing chamber.
Common gear shroud designs operate by limiting the ability of the gears in the gear system to do work on the general air-oil mixture in the bearing chamber by employing restricted inlet and exit areas (i.e. by throttling at the inlet and exit of the shroud). In this way, the quantity of air-oil mixture induced through the shroud may be minimised.
The air-oil mixture that is entrained by the gear system primarily passes along the gear teeth and takes a spiral type route (i.e. with both circumferential and azimuthal velocity components) through the gear-shroud system.
The surface of the gear shroud is usually primarily conic both for simplicity of manufacture and to maintain a uniform clearance over the gear teeth (to avoid highly undesirable mechanical gear to shroud clashes at extreme limits of gear setting and/or thrust bearing internal clearance levels).
In addition to the main inlet and exit areas, some means is also usually provided to scavenge away the gear mesh lubricating oil which is normally introduced from a dedicated oil jet upstream of the mesh point (so called ‘into mesh’ lubrication). An effective scavenge arrangement for the lubricating oil is a slot which extends for the full length of the gear face width and is usually positioned approximately 30 deg past the gear mesh point.
A disadvantage of the prior art is that any oil which does not exit via the intended slot may be caught under the shroud and recirculate within the shroud as shown in FIG. 3. This recirculation arises primarily as a result of the above-mentioned large clearance between gear tooth tip and gear shroud.
The region of flow recirculation commonly extends over the entire width of the face of the gear, from almost the shroud inlet to almost the shroud exhaust. This large recirculation flow region is undesirable as it can result in locally excessive oil temperatures resulting from the heat generated by the additional pumping work of the gear system.
Such locally excessive oil temperatures may result in oil degradation (or even combustion) and the possible requirement for the provision of additional oil coolers in an engine heat management system where the oil system is essentially closed.